Client should read capabilities instead of sniffing server version

custom builds of Couchbase Server sometimes return invalid version strings like 0.0.0. The DCP client does not behave well when connecting to such a server. Instead of using the version string to decide which features to enable, it should get the information from the `hello` or `select_bucket` responses.

The client currently only issues a `hello` request if the server version is < 5.0.0. Trond points out that the `hello` command is present at least as far back as 4.0.0, so maybe we should unconditionally issue the hello request.